Margaret Thatcher said there was "no question" of agreeing to give Dublin joint authority over the running of Northern Ireland, official files reveal.

The former Prime Minister told unionist leaders that the Anglo-Irish Agreement between the British and Irish governments did not threaten UK sovereignty. Her intervention followed a long-running unionist campaign against the 1985 Agreement, which gave the Irish Government a consultative role in Northern Ireland for the first time.

The following year Mrs Thatcher wrote: "It is expressly stated in the Agreement that there is no derogation from the sovereignty of the UK Government, which remains solely responsible for decisions in relation to the affairs of Northern Ireland.

"There is consequently no question of 'joint authority', nor does the Agreement in any way threaten the Union; I should not have been a party to it if it did."